Catheter systems and stent delivery systems are similar, to the extent that each is usually deployed by advancing it along a guidewire. Not infrequently, there will be exchanges of such devices while the guidewire remains in place. Systems which are categorised as “over the wire” have a guidewire lumen which runs the full length of the device. As this is typically of the order of 130 cm, it can be cumbersome and inconvenient to exchange for a different device while the guidewire remains in place, leading to the site of surgical treatment.
By contrast, the so-called “rapid exchange” catheter systems have a guidewire lumen which runs much less than the full length of the system, and only in a distal portion of the length. In such systems, the length of guidewire protruding from the body of the patient during exchange of one catheter system for another, over the same guidewire, need be only a fraction of the full length of the catheter system, making it easier and quicker to make the exchange. This is a good reason for preferring rapid exchange systems to “over-the-wire” systems, other things being equal.
An early example of a rapid exchange catheter system is to be found in EP-A-0 380 873. The system shows a balloon catheter with a substantial part of its length defined by a hypotube which delivers inflation fluid to a balloon at the distal end of the catheter. Running through the balloon is a tube defining a guidewire lumen extending from a distal exit port distal of the balloon to a proximal guidewire exit port some distance proximal of the balloon. Proximal of the proximal guidewire port, the guidewire runs parallel to the hypotube shaft, but outside it. Advancing the catheter device over the guidewire is relatively simple, because the guidewire lumen is more or less straight, with a continuous smooth luminal wall surface.
Reference is now made to the stent delivery system disclosed in EP 505 686. It is to be noticed that the path of the guidewire through the stent delivery system of EP 505 686 is significantly more complicated than that of EP 380 873. Not only does the guidewire pass through the cylindrical wall of the catheter body on which the balloon is mounted, but also through the cylindrical wall of a sheath element which surrounds the stent and which is retracted, relative to the catheter body, for deployment of the stent. To accommodate the guidewire during stent deployment, the sheath 10 has not only a proximal guidewire exit port but also a long slit, the walls of which slide over the guidewire during proximal retraction of the sheath. Furthermore, to deflect the guidewire out of the proximal exit port in the cylindrical wall of the catheter body, there is provided a plug with a tapered end surface which will guide the proximal end of the guidewire, during backloading of the catheter device onto the guidewire, out of the lumen of the catheter body, through the proximal guidewire exit port.